This invention relates to a copolyimide film having low water absorption, low coefficients of hygroscopic and thermal expansion, enhanced etchability, and increased modulus, particularly useful for use as a tape automated bonding (TAB) substrate.
TAB is an electronic chip packaging technology which uses an etched, fine-line conductor pattern in a flexible carrier tape form.
The common construction of the tape carrier/interconnect product is virtually identical to flexible circuitry, except for two special features. An opening, appropriately called a window, is formed near the center of the conductor array in the dielectric base film. The window permits the etched conductor leads to extend over the opening, creating the essential beam type interconnect array.
The other characteristic of TAB is the precision sprocket holes located on the edge of the tape. TAB tape is provided in reel form, looking much like movie film and is available in widths from 8 to 70 mm.
The integrated circuit (IC) connection to TAB is accomplished by placing the chip in the window area beneath the unsupported connectors, or "fingers", aligning the leads with the metal-coated bonding pads on the chip, and then gang bonding the entire array by thermal compression. A raised metal area, called a "bump", must be incorporated into the IC pads or into the TAB leads to make the bonding process workable. This gang bonding technique, called inner lead bonding (ILB), provides two important features, viz. assembly speed and connection to higher density chips. The bonded IC is usually protected by applying organic potting compound onto the chip for protection in a process called "globbing" or "encapsulation". The loaded tape is then assembled to the printed circuit.
The subsequent TAB-to-circuit bonding step, called outer lead bonding (OLB) requires that the TAB interconnect area be etched from the tape. The etched TAB component is bonded to the surface of the printed circuit by aligning the TAB outer lead frame to corresponding bond sites on the circuit and then applying bonding energy by thermal compression or reflow soldering. The resulting assembly occupies a smaller space, has a very low profile and has superior electrical characteristics compared to its popular wire-bonded counterpart.
Three-layer TAB tapes are the most widely used today. This type of tape consists of copper foil which is often 1.4 mils thick and is bonded to a polyimide film with a layer of polyester, acrylic, or epoxy based adhesive. Normally, the polyimide film is coated on one side with adhesive and slit to the correct width. The windows and sprocket holes are punched or chemically etched out and the tape is then laminated to the copper foil. The foil is then selectively etched into the radiating patterns that form the IC interconnects.
Two-layer TAB tapes are becoming more functionally useful and consist of copper bonded directly to polyimide, without a layer of polyester, acrylic, or epoxy based adhesive. Most often, the copper is applied to the polyimide by sputtering or electroless plating and built to a thick, 1.4 mil layer of copper by electroplating. Sprocket holes can be punched, but windows are etched in the polyimide.
The polyimide used as the base film in TAB tapes must have a relatively small thermal expansion coefficient which is close to the thermal expansion coefficient of the metallic film, otherwise curling of the composite tape occurs during exposure to high temperatures during processing. The film must have good flexibility to avoid cracking on TAB equipment and high modulus so that sprocket holes do not distort during movement of the tape.
The film must also have a low coefficient of hygroscopic expansion so that dimensions stay more constant with humidity changes, thereby making humidity control less important in processing and causing less stress to build up in humidity cycling to prevent cracking. Finally, film etchability allows chemical etching of holes in the dielectric rather than punching them or using laser cutting.
U.S. Pat. No. 4,778,872 issued to Sasaki et al. on Oct. 18, 1988, discloses a copolyimide film derived from biphenyltetracarboxylic acid dianhydride, pyromellitic acid dianhydride and aromatic diamines prepared by a thermal conversion process and having a relatively small thermal expansion coefficient, high mechanical strength and good flexibility. However, this thermally converted copolyimide film has a high coefficient of hygroscopic expansion and is not readily etchable making it unsuitable for use as a base film in a TAB tape.
It has now been found that a copolyimide film derived from biphenyltetracarboxylic acid dianhydride, pyromellitic acid dianhydride, diaminodiphenylether and p-phenylenediamine prepared by a chemical conversion process provides advantages compared to copolyimide film prepared by a thermal conversion process. These advantages include enhanced chemical (caustic) etchability up to 400.degree. C. cure, reduced coefficient of hygroscopic expansion, reduced water absorption, lower thermal coefficient of expansion and higher modulus.